Okay, this time you're serious. You're going on a regimen that will really improve your health—not like that crash diet: You'll snarf down antioxidants; exercise moderately but consistently; balance fats, proteins, and carbs; and pay attention to the way you explain whatever happens to you.

Wait a minute. The way you explain what happens? What does that have to do with physical health? According to findings from the burgeoning field of behavioral medicine, a lot. How we think can affect physical processes as surely as diet and exercise do. For example, putting a positive spin on events in our past is associated with an enormous array of health benefits, from improved immune function to reduced stress to quicker healing, with all their emotional and physical advantages. To some degree, we may be able to literally explain away many devastating physical problems. If you want to have a healthier body, I suggest changing your mind first.

So What's Your Story?

Caroline, one of my brightest, prettiest, best-educated clients, was a wreck. Her pet cockatiel, Bonkers, had flown away.

The way she told the story of her bird's disappearance—what researchers have called "explanatory style"—was making her situation much worse. Her explanation of Bonkers' great escape showed the three key markers of pessimism: She described the problem as being personal ("I made it happen; these things always happen to me"), permanent ("Things will never get better"), and pervasive ("My whole life is rotten; I'm such a loser").

On the other hand, I'd noticed that whenever something good happened, she explained it as a fluke. " This cute guy from work asked me out," she said one day. Caroline explained the man's interest in her as his own "insanity" (not personal) and assumed it wouldn't last (not permanent). She stressed that other people's interest never lasted, even though I knew she had been the one to end most romantic relationships (not pervasive).

Bear in mind that Caroline didn't think like this only when she talked to me. Day in, day out, her mind serialized every piece of bad luck into another episode in a continuing Saga of Doom and deflected every happy event into the Meaningless Trivia scrap pile. Her style was crushing her mood—and was probably damaging her body as well.

Why See the Glass Half Empty?

Despite its attendant miseries, there seems to be a useful place for a pessimistic explanatory style. Some people appear to downplay positive aspects of their situations to limit their expectations and help them feel less pressured. They're less likely to feel let down if things go wrong.

Researchers Julie Norem and Nancy Cantor call this defensive pessimism. My friend Julia calls it inoculating yourself against disappointment. In the seven years I've known her, Julia has changed her explanatory style deliberately, gradually, and successfully. Giving up defensive pessimism may invite disappointment in certain situations, but overall, Julia's quality of life and her physical health are benefiting as she turns herself into a thoroughgoing optimist.

This doesn't happen overnight. If you're a habitual pessimist, you know there's nothing worse than those bouncy optimists.

Habitual thought patterns are like ruts in a dirt road. The mind slips into them over and over, and at first, steering down another route is extremely difficult. Stopping habitual thoughts as they flash along these pathways, turning one's mental energy to a new way of thinking, requires an effort that is not merely impressive but heroic.